150 ELEMENTS. OF STATIC ELECTRICITY. 



force exerted on them, as shown when they are free to 

 move. 



Fig. 45 shows such a tube, having a glass railway on 

 which is placed a roller with mica vanes, and the 

 electrodes so placed that the upper vanes are, in line 

 between them. When an electric current passes 

 through the tube, these vanes, being at zero potential, 

 are attracted by the higher potential of the positive 

 electrode, producing a rotary movement of the roller 

 from negative to positive ; the force being sufficient to 

 move it up an incline. 



Fig. 46 Rotary Movement Reversed. 



Fig. 46 shows a tube in which a wheel with mica 

 vanes is so mounted that its center is in line between 

 the positive electrode, and the center of the negative. 

 The negative electrode a b is cup-shaped, and its con- 

 cave surface turned towards the positive : so that the 

 lines of force may be brought to a focus, and concen- 

 trated on the vanes. And between its center and the 

 wheel is placed the mica screen c d. 



A magnet, #, is suspended above the tube, between 



